With the explosion of demand for a mass fever scanning system to detect high body temperature with touchless IR thermometer, several fever screening devices have flooded the market. Since this market is rapidly evolving, exhaustive testing to evaluate the merit and demerit of the different types of Fever screening methods is grossly lacking. At this point, one can only refer to the scientific and exhaustive works that have been done in the past in the area of body temperature measurement using IR methods.
Fever is defined by an increase in Mean Body Temperature (MBT). Skin body temperature and core-body temperature differ by a few degrees of centigrade [skin body temperature will be 2-4C lower than core body temperature] but they do correlate well with the increase or decrease of Fever  . Details and exhaustive studies show skin temperature of different parts of the body such as hand, legs, feet, belly, chest, forehead, etc. may not be the same but they correlate well with the increase or decrease of MBT  . Detailed studies have shown IR measurement of the fist ( hand skin) is reliable and in fact, can be used not only for Fever but also for more complex diseases like STA ( Skin Temperature Asymmetries)  .
After studying the advantages and disadvantages of several methods of mass fever scanning, MachineSense has selected the IR temperature scanning of the fist within close distance of 1-4 inch as the most ideal method when considered from logistics, reliability and cost points of the view.
Considering the practicality and logistics of mass Fever Screening, only face and fist are the available exposed skin segment for the screening during all seasons. Rest of the body will be covered and thus can’t be useful at all for mass-screening. There can be an exception to this rule due to religious dress codes and winter seasons respectively, but the only way to detect fever quickly can happen by scanning face or fist. IR measurement of some segments of Face and Fist correlate well with body temperature. However, for automated fever screening, measuring Fist skin temperature is easier, cheaper and reliable.
The forehead can be automatically scanned only by an array of IR sensor or IR Imaging system. This is due to the fact that all of us have different heights and therefore, only an IR imaging gun which can work with at least 3 ft of range can scan different portion of the skin temperature reliably. However, this creates multiple issues:
- Long-range IR imaging systems are expensive and in the range of $3000-$30,000. One has to use very high image resolution IR camera [ at least 512 pixels or so] because only a few small areas of the face correlate well with body temperature 
- Uniform Calibration of such a system within 0.5C is difficult since very often people will be using different kinds of creams, ointments, make-up on the face making it’s emissivity much lower than human skin ( 0.9-0.95)  . For example, metal power oxide based make-up may reduce the emissivity of the face to a 0.5-0.6 range. Since these systems are factory calibrated for the emissivity of 0.90-.99, face with make-up and ointments will give erroneous results. Besides even within the face, there are areas which do not correlate with core-body temperature. 
- IR imaging system can’t be used outdoor. IR imaging works only indoor since LED or indoor light doesn’t have IR components. Sunlight has a strong IR presence (2-10 micron) and thus IR image scanners do not work outdoor. In many of the factories and offices, Fever scanning is preferably done outside the gate entrance. The rationale is justified because if a person with fever enters the premises, the virus will spread through its centralized HVAC system. Besides, in a school bus, commuter trains - this must be done outside the gate before a passenger is admitted inside the commuter system.
Considering the practical difficulty of 1-3 for Face scanning, MachineSense decided to develop fist scanning as the optimized method for mass Fever Scanning. The skin of the Fist gets one of the best blood circulations as it is the distribution center of blood supply for the fingers. Increased body temperatures spread to the skin via blood circulation only. As shown in  , skins of the hand respond to an increase or decrease of core body temperature reliably. Fist temperature can be accurately measured by a single IR sensor with 0.2C and the result is repeatable within 0.3C as can be shown from this test result. This makes the system reliable and cheaper.
However, one has to understand IR skin temperature screening has its own limitation no matter how expensive an IR system is used. Skins whether be it a fist or forehead will be affected if a person is coming from extreme winter or cold weather or soaked with water and covered in ointment. Both Forehead and Fist screening suffer from this limitation of being affected by extreme cold and hot weather conditioning of the skin. For those cases, the only solution is to wait 5-10 minutes in moderate ambient conditions before the screening. Or adopt a more advanced Fever Screening model that will include heartbeat, pulse rate, etc., along with body temperature to detect Fever.
MachineSense has released such a system called FeverWarn 200 and FeverWarn 300 in May 2020.
 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1752199/  https://viterbik12.usc.edu/wp-content/uploads/2017/06/2012-Building-and-Environment-Choi2c-Loftness-Investigation-of-human-body-skin-temperatures-as-a-bio-signal-to-indicate-overall-the.pdf  https://pubmed.ncbi.nlm.nih.gov/22975739/  https://www.fierceelectronics.com/sensors/crowd-temp-screening-a-joke-one-tech-expert-says  https://pubmed.ncbi.nlm.nih.gov/23084004/